It is generally recognized that fossil fuels are limited and that the conventional use of petrochemical resources contributes to environmental effects that impact our global environment. It is clear that a new, sustainable technology that is based on renewable resources has to be developed. One technology that is being as an alternative to petroleum transportation fuels developed is the production of ethanol and other energy carriers from renewable feedstocks such as starch and cellulose.
The hydrolysis of starch or lignocellulosic feedstocks typically result in 6-carbon and 5-carbon sugar precursor mixtures that must be enzymatically converted into ethanol or other valuable energy carriers. The enzymatic conversion can be performed by a number of different microorganisms. Native microorganisms, however, typically have not evolved to carry out desired conversions at the best possible yield. Furthermore, inhibitory substances generated during the hydrolysis steps often negatively affect the microorganisms that are generating ethanol from the sugars.
Thus, efficient and robust microorganisms that are able to function under adverse conditions are needed to establish an optimal and cost efficient sugar-to-ethanol conversion technology. Because different sugar-containing substrates can contain a combination of different sugar precursors, the conditions under which efficient conversion of one sugar-containing substrate into ethanol takes place (e.g., the particular microorganism(s), the particular culture conditions, and the particular inoculum of the microorganism(s)) may be different than those conditions under which efficient conversion of a different sugar-containing substrate into ethanol takes place.